This invention relates to a coating system which forms a highly abrasion resistant, chemical resistant, impact resistant protective finish for a substrate.
Coating compositions have been developed which, when applied to a substrate and cured, impart a highly abrasion resistant surface to the substrate. Coating compositions of this type have been widely used to impart abrasion resistance to plastic lenses such as eyeglass lenses, to plastic panels and films, to wood surfaces such as furniture, and many other applications where an abrasion resistant or scratch resistant surface finish is of importance.
Abrasion resistant coatings of this type are typically based upon acrylate monomers which are cured or crosslinked after application of the coating, typically by radiation curing. Radiation curable coatings offer the advantage of being rapidly cured and polymerized without requiring curing ovens and they can be applied and processed without having to remove solvents and deal with solvent vapors in the workplace environment.
It is known that radiation cured acrylate polymers can produce very hard (glass hard) protective coatings which exhibit superior abrasion and chemical resistance properties. Although the coatings are quite hard and resistant to abrasion and scratching, they are brittle and have a tendency to crack and peel from the substrate, especially when applied to relatively flexible substrates or when subjected to impact.
Prior abrasion resistant coatings have sought to deal with the brittleness and cracking problem by using a softening comonomer (a monomer with a low second order transition temperature) to impart some degree of flexibility to the coating. However, in achieving increased flexibility and reduced brittleness, the abrasion resistance of the coating is sacrificed. Thus, for example, U.S. Pat. No. 4,319,811 discloses an abrasion resistant radiation curable coating based upon tri- and tetraacrylate monomers, such as pentaerythritol triacrylate with a comonomer such as vinyl pyrrolidone or vinyl caprolactam. U.S. Pat. No. 4,308,119 discloses an abrasion resistant radiation curable coating composition comprised of a pentaerythritol tetraacrylate with a cellulose ester such as cellulose acetate butyrate. U.S. Pat. No. 4,557,980 discloses a radiation curable coating composition based upon a mixture of a triacrylate or tetraacrylate, such as pentaerythritol tetraacrylate, with acrylic acid.
The resistance of a coating to scratching abrasion is typically measured by the rotary steel wool test, which involves subjecting the coating to five revolutions of a pad of 0000 grade steel wool at a defined pressure, usually 12 or 24 psi. The scratching abrasion resistance is rated by measuring the increase in haze from the abrasion. Test methods such as ASTM D-1044 have been developed for optically measuring the resistance of transparent plastic materials to abrasion. Other standard tests for abrasion resistance are the Taber abrasion test described in ASTM D-1004-56.
In many applications, the protective finish needs not only to be "hard" and thus resistant to scratching, but also must have excellent toughness and resistance to impact. The toughness or impact abrasion resistance of a coating is commonly measured by the "falling sand" test (ASTM D968-51). A coating which has good scratch abrasion resistance may not necessarily have good impact abrasion resistance. With the falling sand test, sand is poured onto a coating from a predetermined height, while the thickness of the coating is observed. The results are expressed in terms of the number of liters of sand required to abrade away one tenth of a mil of the coating thickness. The radiation cured abrasion resistance coatings noted in the aforementioned prior patents have a relatively poor resistance to impact abrasion which renders these types of coatings unacceptable for applications requiring both good resistance to scratching abrasion and good resistance to impact abrasion.
With the foregoing in mind, it is an important object of the present invention to provide a coating system which provides not only good resistance to scratching abrasion, but also provides excellent resistance to impact abrasion.